Method used in gas-shielded metal-arc welding

ABSTRACT

The invention relates a method for gas-shielded metal-arc welding during the re-strike during the welding process. It is object of the invention to provide a gas-shielded metal-arc welding method to permit as low an energy intake as possible in the short-circuit phase for the arc welding. According to the invention, this object is solved by a method, characterized in that at the beginning of a short-circuit, a control is activated and remains active for the duration of the short-circuit, which increases an energy intake (phase A) when a characteristic threshold value S 1  is fallen short of, and terminates the energy intake when a threshold value S 2  is reached, and than lowers the energy intake (phase B).

The invention concerns a method used in gas-shielded metal arc weldingin accordance with the preamble of patent claim 1.

Gas-shielded metal-arc welding (MSG welding) has been used for a longtime for hard facing, welding or soldering of two or more pieces to bejoint, made of metallic materials. In an inert gas atmosphere, thefiller material in form of a wire or a band is melted in an arc createdby an electric welding power source. The arc, in this case, burnsbetween the base material and melting filler material (electrode).Depending on the inert gas used, one speaks of metal active gas welding(MAG welding) or metal inert gas welding (MIG welding). Furthercharacteristics depend on the choice of different processing parameters.Thus one differentiates between short (circuiting) arc welding, long arcwelding, spray arc welding, rotation arc welding and pulsed arc welding.

Short-circuiting arc welding (Klb welding) is characterized by an arcillumination phase and a short-circuit phase. In the arc illuminationphase, a melted drop is created at the electrode end. Due to thecontinuous wire supply and the increasing drop volume, the drop touchesthe melting bath after a certain time. In this short-circuit phase, thedrop is tied off by the high short-circuit current that sets in and thearc strikes again. The welding process thus changes back and forth inmore or less regular intervals between two process states, wherein thematerial transmission takes place exclusively in the short-circuitphase.

In this cyclical welding process, stochastic fluctuations of the processparameters occur, which are caused by external disturbances and by thecharacteristic of the process feedback control in the machine. In thisprocess, the short-circuit duration, and usually also the amount ofcurrent when a short-circuit breaks and the arc re-strikes, cannot bedefined.

In short-circuiting arc welding processes up to date, the currentcontinuously increases with time (dt) in the short-circuit phaseaccording to the machine characteristics. In newer machines, differentdI/dt are possible for the current rise in the short-circuit phase,sometimes with several phases. The short-circuit treatment in pulsed arcwelding is similar to this. During the breaking the short-circuit andthe following arc strike a substantially higher current thereforeexists.

The main disadvantage of a high re-strike current is the largetemperature rise of the melted bridge between the electrode wire and themelting bath, which leads to sudden breaking.

As a consequence, problems occur in the welding result, such as:

-   -   Welding splatters    -   Blowing out of the melt with a hole formation, in particular for        thin sheets    -   Evaporation losses, in particular for alloy elements with high        vapor pressure, such as Zn and Mg

In DE 41 29 247 A1, the STT method is described, which recognizes theupcoming short-circuit breaking by a measurement of the potentialgradient dU/dt. When exceeding a limit value, the current is reduced to50 A for a few microseconds before breaking.

Disadvantages of this method are on one hand very complex signalprocessing of the potential measurement, which is strongly disturbed byelectromagnetic fields during the welding process and therefore limitsthe signal sensitivity. On the other hand, the inductance of the weldingenergy source limits the speed of the reversal and reduction of thecurrent so that at a high level during the breaking, a very high energyintake still takes place.

It is object of the invention to provide a metal inert gas arc weldingmethod to permit as low an energy intake as possible in theshort-circuit phase for the arc welding.

According to the invention, this task is solved by the characteristicfeatures of claim 1. Advantageous derivatives are listed in thedependent claims.

By means of a definition of threshold values in the short-circuit phase,the properties of the welding energy source is achieved, so that on theone hand the energy intake has as low a value as possible when the shortcircuiting bridge breaks and the arc re-strikes and on the other handthe energy intake drops as fast as possible. Besides, the forthcomingbreaking of the bridge can be recognized before it occurs.

The method according to the invention is now characterized by the factthat, at the beginning of a short-circuit, a control is activated andremains active for the duration of the short-circuit, which increases anenergy intake (phase A) when a characteristic threshold value S1 isfallen short of, and terminates the energy intake when a threshold valueS2 is reached, and than lowers the energy intake (phase B).

The threshold value S1 may be a voltage, and the threshold value S2 maybe a voltage, a current, a resistance, a power, or any other suitablethreshold value.

In a derivative of the invention, the two phases A and B repeat untilthe short-circuit is broken, wherein the sampling rate of the thresholdvalues, and thus the duration of the phase, are adaptable in phase A andphase B.

The breaking of the short circuiting bridge, as well as the followingstrike of the arc, always take place in phase B. The energy rise inphase A and the energy drop in phase B are arbitrary in this case.

In a further embodiment of the invention, the energy rise in phase A andthe energy drop in phase B are described by a polynomial, exponential,trigonometric, cyclometric or hyperbolic function.

In a further derivative of this case, the energy rise in phase A and theenergy drop in phase B is described by a combination or succession offunctions.

The course in time of the energy intake before and after theshort-circuit is arbitrary.

In a further embodiment of the invention, the course of the energyintake before and after the short-circuit is represented by apolynomial, exponential, trigonometric, cyclometric or hyperbolicfunction. In a derivative, the course in time of the energy intakebefore and after the short-circuit can be described by a combinationand/or succession of functions.

According to the invention, the transition from one to another functionis triggered by a time criterion and/or by an evaluation of one or morewelding process signals, wherein the process signals can be evaluated bymeans of a neural system.

The transition from one to another function is triggered according tothe invention through a logical link of criteria.

Such a welding process signal is the welding potential, the weldingcurrent, or any process variable measurable by a sensor, wherein theprocess variable is a radiation, sound, an electric field or a magneticfield.

In another embodiment of the invention, the welding energy source iscurrent-controlled, and preferably also potential, power, orresistance-controlled.

The welding energy source operates in different process phases withdifferent controls.

The energy intake adapts itself to the course of the materialtransmission. It is self-regulating and is reduced to a minimum when theshort-circuit breaks. The welding process becomes more uniform andsmoother. Splatter creation no longer occurs.

The re-strike of the arc no longer takes place suddenly and, inparticular for thin sheet welding, the melt is no longer ejected.

The evaporation of electrode material is substantially reduced.

Furthermore, very light sheet metals can be welded without problems. Forsurface refines sheet metals, the danger of the insufficient degassingis reduced and the melting loss of the refinement layer is very low.

Embodiments of the invention are shown in the drawings and described indetail in the following.

It is shown in

FIG. 1 a schematic current and potential course of a short-circuitingarc welding process,

FIG. 2 a schematic current and potential course of a short-circuit forpulsed arc welding process, and

FIG. 3 a current and potential gradient according to the invention inthe short-circuit phase.

In short-circuiting arc welding in accordance with FIG. 1, a continuouschange between an arc illumination phase and a short-circuit phaseoccurs, as is well-known. In the short-circuit phase the arc goes outand a melted fusion between the melting electrode and the melting bathon the workpiece is created. As can be seen in the current course inFIG. 1, the current continuously rises from time A, so that whenbreaking the short circuiting bridge and igniting the arc between c andd, a high energy intake takes place.

This also occurs occasionally during pulsed arc welding according toFIG. 2, which normally is short-circuit-free.

In FIG. 3, an embodiment of the general course of current and voltage isshown. The threshold value S1 is a voltage value, and the thresholdvalue S2 is a current value.

1. A method used in gas-shielded metal-arc welding during the restrikeduring the welding process, comprising the steps of: activating acontrol at a beginning of a short-circuit which remains active for theduration of the short-circuit and increasing an energy intake (phase A)when a characteristic threshold value S1 is fallen short of andterminating the energy intake when a threshold value S2 is reached, andlowering the energy intake (phase B) thereafter.
 2. The method accordingto claim 1, further comprising the steps of selecting a voltage value asthe threshold value S1 and selecting at least one of a voltage, acurrent, a resistance and a power value as the threshold value S2. 3.The method according to claim 1, further comprising the step ofrepeating both phase A and phase B until the short-circuit is broken. 4.The method according to claim 3, further comprising the step of changingthe sampling rate of the threshold value, and thus the duration of phaseA and phase B.
 5. The method according to claim 1, further comprisingthe steps of always breaking the short circuiting bridge and strikingthe arc in phase B.
 6. The method according to claim 5, furthercomprising the steps of raising the energy intake in phase A by anarbitrary amount and dropping the energy intake in phase B by anarbitrary amount.
 7. The method according to claim 1, further comprisingthe step of describing the energy intake rise in phase A and the energyintake drop in phase B by at least one of a polynominal, exponential,trigonometric, cyclometric and hyperbolic function.
 8. The methodaccording to claim 7, further comprising the step of describing theenergy intake rise in phase A and the energy intake drop in phase B byat least one of a combination and a succession of functions.
 9. Themethod according to claim 1, further comprising the step of adjustingthe energy intake before and after the short-circuit to be an arbitraryfunction of time.
 10. The method according to claim 9, wherein said stepof adjusting the energy intake before and after the short-circuitincludes using at least one of a polynomial, exponential, trigonometric,cyclometric, and hyperbolic function.
 11. The method according to claim1, further comprising the step of adjusting the energy intake before andafter the short-circuit by using at least one of a combination andsuccession of functions.
 12. The method according to claim 11, furthercomprising the step of triggering the transition from one function toanother function by at least one of a time criterion and an evaluationof at least one of the welding process signals.
 13. The method accordingto claim 1, further comprising the step of selecting at least one of awelding potential, a welding current, and any process variablemeasurable by a sensor for such a welding process signal.
 14. The methodaccording to claim 13, wherein said step of selecting any processvariables includes selecting the process variable from at least one of aradiation property, a sound property, an electric field property, and amagnetic field property.
 15. The method according to claim 1, furthercomprising the step of current-controlling a welding energy source. 16.The method according to claim 15, further comprising the step ofselecting the welding energy source from at least one of a potentialsource, a power source, and a resistance-controlled source.
 17. Themethod according to claim 1, further comprising the step of regulating awelding energy source in different process phases with differentcontrols.